The present invention relates to methods for preparing sputter target/backing plate assemblies, and to the target/backing plate assemblies prepared by these methods. More particularly, the invention pertains to methods for diffusion bonding high purity metal or metal alloy sputter targets to associated backing plates composed of aluminum, aluminum alloy or aluminum matrix composite material, using nickel interlayers and to assemblies produced thereby.
Cathodic sputtering is widely used for depositing thin layers or films of materials from sputter targets onto desired substrates. Basically, a cathode assembly including the sputter target is placed together with an anode in a chamber filled with an inert gas, preferably argon. The desired substrate is positioned in the chamber near the anode with a receiving surface oriented normally to a path between the cathode assembly and the anode. A high voltage electric field is applied across the cathode assembly and the anode.
Electrons ejected from the cathode assembly ionize the inert gas. The electrical field then propels positively charged ions of the inert gas against a sputtering surface of the sputter target. Material dislodged from the sputter target by the ion bombardment traverses the chamber and deposits to form the thin layer or film on the receiving surface of the substrate.
In so-called magnetron sputtering, a magnet is positioned behind the cathode assembly to generate an arch-shaped magnetic field. The arch-shaped field, formed in a closed loop configuration over the sputtering surface of the sputter target, serves to trap electrons in annular regions adjacent the sputtering surface. The increased concentrations of electrons in the annular regions adjacent the sputtering surface promote the ionization of the inert gas in those regions and increase the frequency with which the gas ions strike the sputtering surface beneath those regions.
The sputter target is heated during the sputtering process by the thermal energy of the bombarding gas ions. This heat is dissipated by heat exchange with a cooling fluid typically circulated beneath or around a backing plate which is bonded to the sputter target along an interface opposite the sputtering surface.
High purity metal and metal alloy sputter targets historically have been solder bonded to copper alloy backing plates. Yet, as larger and heavier sputtering targets have been used in coating progressively larger substrates, mechanical problems, such as deflection of the sputter target/backing plate assembly during use, and ergonomic problems, such as difficulty in lifting the assembly during manufacture and in installing the assembly for use, have arisen. The relatively high specific gravities of copper and copper alloys have led to efforts to manufacture backing plates from lighter metals such as aluminum, aluminum alloys and aluminum matrix composite materials in order to reduce the weights of sputter target/backing plate assemblies. Where an assembly comprising a copper sputter target and a copper backing plate might weigh approximately 40-50 lbs. (18-23 kg.), an assembly using a copper sputter target and an aluminum or aluminum alloy backing plate of the same sizes may weigh only 25-30 lbs. (11-14 kg.).
Furthermore, the use of larger sputter targets and higher levels of sputtering power has increased the stresses on the bonds joining the sputter targets to the backing plates. To a certain extent, soft solders have accommodated stresses exerted on the sputter target/backing plate assemblies as the assemblies are heated during the sputtering process and subsequently cooled. When weak solder bonds have been used to join materials with widely differing thermal expansion rates, however, the bonds have been susceptible to shear failure initiating at the extreme edges of the bond interfaces. Such shear failures commonly have resulted in debonding of the sputter targets from the backing plates during service. The higher melting temperature solders used for high power applications are stronger but have been less forgiving of the stresses developed in the material systems. As a consequence, the use of solder bonding has imposed practical limits on the sizes of the sputter targets and the levels of sputtering power which sputter target/backing plate assemblies could sustain.
High strength sputter target/backing plate assemblies have been formed by diffusion bonding. A diffusion bond is formed by pressing mating surfaces of the sputter target and the backing plate into intimate contact while heating the sputter target and backing plate materials to a temperature just below their melting points to induce the materials to interdiffuse.
Diffusion bonds generally have required extreme care in preparation and in maintaining surface cleanliness prior to and during the bonding operation to ensure reliable bond qualities. Unless removed, surface oxide layers formed on metals such as aluminum could act as diffusion barriers, increasing the difficulty of forming strong bonds. Typical planar bond interfaces were subject to shearing which commonly led to peeling away at the ends of the bond areas. The formation of brittle intermetallics at the bond interfaces, which increased in thickness with longer periods of heat exposure, added to the potential for bond shear failures.
Mueller et al. U.S. Pat. No. 5,230,459, issued in July of 1993, taught that successful high-strength sputter target/backing plate assemblies could be manufactured by diffusion bonding high purity titanium sputter targets to lightweight aluminum alloy backing plates. Tensile strengths of up to 25 ksi (1.7xc3x97108 N/m2) have been achieved by this method.
Titanium has the intrinsic property of reducing aluminum oxide at elevated temperature, thus preventing the oxide from acting as a diffusion barrier. Furthermore, the addition of microgrooves, machined on a mating surface of the titanium sputter target, acts to puncture the oxide layer as well as to provide additional surface area for diffusion. Aluminum diffuses rapidly into the titanium at the interface, forming a sound metallurgical bond that is sufficiently thick yet not composed of brittle intermetallic phases.
Similar methods have not been successful in forming high-strength metallurgical bonds between sputter targets of other high purity metals and their alloys, such as but not limited to copper, cobalt, palladium and platinum, and backing plates of lightweight aluminum. The respective binary systems between these metals and aluminum exhibit several brittle metallic interphases at low temperatures near the aluminum termini of their phase diagrams. For example, copper and aluminum are capable of forming a low temperature intermetallic phase incapable of withstanding a tensile stress greater than approximately 2 ksi (1.4xc3x97107 N/m2). Thick layers of these brittle interphases tend to form during the diffusion bonding process. This morphology at the interface substantially reduces the mechanical load necessary to initiate failure during tensile testing.
Accordingly, there remains a need in the art for a method for bonding high purity metals and metal alloys to lightweight metallic backing plates so that the bonded joint will be mechanically sound so as to allow for higher levels of sputtering power and larger sputter target designs without risk of deflection or debonding in use at elevated temperatures.
Additionally due to the increasing usage of copper in semiconductor circuitry, there is a need for a light weight copper target/backing plate assembly that can withstand prolonged high power sputter usage periods without significant target/backing plate debonding.
These and other objects of the invention are met by a method for manufacturing or fabricating sputter target/backing plate assemblies in which high purity metal or metal alloy sputter targets are diffusion bonded to backing plates formed from aluminum, aluminum alloy or aluminum matrix composite material through the medium of a nickel or nickel alloy interlayer. The method is of particular utility in connection with the joining of such backing plates to sputter targets formed from metallic materials such as copper, cobalt, palladium and platinum which tend to form low temperature brittle intermetallic phases when diffusion bonded directly to aluminum or its alloys.
More specifically, the invention contemplates an improved method for joining mating surfaces of a metallic sputter target and a backing plate of aluminum, aluminum alloy or aluminum matrix composite material to form a sputter target/backing plate assembly. The method comprises the steps of roughening the mating surface of either the sputter target or the backing plate to form a plurality of salient portions projecting from one of the surfaces; depositing an intermediate layer comprising nickel on one of the mating surfaces; pressing the sputter target and the backing plate together along the mating surfaces so as to disrupt the mating surfaces; and holding the sputter target and the backing plate in contact at a temperature just below the melting points of the sputter target and backing plate materials to promote diffusion bonding.
In an especially preferred form of the invention, the mating surface on the harder of the sputter target or the backing plate is roughened by machining a series of concentric grooves therein. The grooved mating surface is covered by a layer of nickel having a consistent thickness, preferably by means of plating, sputtering, electroless plating or nickel foil or plasma spraying. The sputter target and the backing plate are then joined along the mating surfaces using hot isostatic pressing or vacuum hot pressing.
The process results in a fully joined assembly wherein the sputter target material, preferably high purity metal or metal alloy, is bonded in situ to the backing plate material, preferably aluminum, aluminum alloy or aluminum matrix composite material, through the medium of the nickel interlayer. Preferably, the nickel interlayer is of sufficient thickness that the sputter target and the backing plate form substantially separate metallurgical bonds with the nickel; that is, the assembly forms two relatively strong intermetallic phases, including a first intermetallic phase between the nickel and the aluminum species of the backing plate and a second intermetallic phase between the nickel and the high purity metal or metal alloy of the sputter target, rather than a single, relatively brittle intermetallic phase between the sputter target and backing plate materials themselves.
Typically, the high purity metal or metal alloy of the sputter target will be harder than the aluminous material of the backing plate. This is true where the target material is copper, cobalt, palladium, platinum or an alloy thereof. When the grooves and the nickel interlayer are formed on a mating surface of a sputter target formed of a metallic material harder than the metallic material from which the backing plate is composed, the jagged salient portions or ridges defined between the grooves serve to puncture the native oxide on the aluminum backing plate. The sides of the grooves provide additional surface area across which the aluminum species from the backing plate diffuse to form a thick, yet strong, nickel-aluminum intermetallic phase.
The disruption of the mating surfaces caused by the pressing of the roughened surface against a substantially planar mating surface is believed to provide additional mechanical advantages. It is believed that the non-planar interface between the sputter target and backing plate portion of the completed assembly increases the resistance of the bond to shear failure. Furthermore, it is believed that the salient portions defined by the grooves fold over during the pressure consolidation of the assembly to form a locking type mechanism.
Whether on the mating surface of the sputter target or of the backing plate, the roughening provides the added advantage of facilitating the deposit of the nickel interlayer through the removal or disruption of any oxide layer formed on that surface.
The interlayer is preferably formed of metallic nickel due to the ability of metallic nickel to form strong intermetallic phases with aluminum. Apart from this, the high electrical conductivity and favorable magnetic properties of nickel minimize interference with the current flows and magnetic fields generated during magnetron sputtering processes. Pass-through flux tests have shown that the magnetic field penetrating the target assembly has substantially the same value as that detected when the same arrangement of magnets is positioned in open air.